〔Abstract〕 Objective To investigate the relationship between neutrophils extracellular net(NET) and follicular helper T cells(Tfh) in patients with anti-neutrophil cytoplasmic myeloperoxidase antibody-associated vasculitis(MPO-AAV) and its significance in the pathogenesis. Methods 35 patients with untreated MPO-AAV(patient group) and 35 healthy volunteersas control(control group) were enrolled in this study.Flow cytometry(FCM) was employed to detect percentage of Tfh in CD4+T cells(Tfh%), percentage of Tfh expressing ICOS in CD4+T cells(ICOS+Tfh%) and mean intensity(MFI) of Tfh expressing ICOS in their peripheral blood, while enzyme linked immunosorbent assay(ELISA) was used to measure the level of MPO-ANCA,NET and IL-23 intheirblood. Their demographic documents were collected and the disease activity of all patients were recorded by Birmingham Vasculitis Activity Score-version 3(BVAS-V3). Results Compared with those in control group, the detected Tfh%,ICOS+Tfh%, MFI, NET and IL-23 increased[(25.7±3.9)%vs(20.7±5.3)%,P<0.001;(1.9±1.0)%vs(0.8±0.4)%,P<0.001;(59.5±10.3)vs(48.7±6.4)(MFI),P<0.001;(0.6±0.2)vs(0.2±0.1)(absorbance),P<0.001; and(0.753 5±0.201 9)vs(0.237 5±0.087 0)(pg/L),P<0.001, respectively].By bivariate correlation analysis, the MPO-ANCA had positive correlation with the Tfh% and the MFI of ICOS+Tfh(r=0.737,P<0.001;r=0.628,P<0.001, respectively), whereas, multivariate linear regression analysis indicated the MPO-ANCA only was associated with Tfh%(P<0.001).Statistical analysis also showed IL-23 was positively correlated with Tfh%, the MFI of ICOS+Tfh, and the NET(r=0.475,P=0.004;r=0.359,P=0.034; andr=0.806,P<0.001, respectively). Although the Tfh% was positively correlated with serum NET and IL-23(r=0.714,P<0.001 andr=0.480,P=0.004, respectively) by bivariate correlation analysis, multivariate linear regression analysis demonstrated that the NET levelwasthe only independent influence factor on Tfh%(P<0.001).By bivariate correlation analysis, the renal lesions had positive correlation with the NET(r=0.390,P=0.021). Conclusion To participate in the pathogenesis of MPO-AAV, NET may increase Tfh% by activating myeloid dendritic cells to assist B cells to produce specific autoantibody MPO-ANCA.